1. Field of the Invention
This invention relates generally to a broadcast communications system, and, more particularly, to a vehicular broadcast communications system that transmits user data with extra protection to improve a channel estimate used to decode the user data with normal protection.
2. Discussion of the Related Art
As the automobile has become more technologically advanced, need has arisen for a reliable vehicular communications network composed of vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication. Applications for a vehicular communications network range from safety to multimedia. By forming vehicular networks, vehicles can share traffic flow information, alert vehicles of hazardous road conditions ahead, and help drivers be more aware of neighboring vehicles. In addition, a reliable vehicular communications network helps enable autonomous vehicles.
The IEEE 802.11p standard, “the standard,” is the core technology for vehicular networks. This standard has a physical layer, which utilizes Orthogonal Frequency Division Multiplexing (OFDM), where OFDM is a spectrally efficient multi-carrier modulation scheme. The subcarriers within an OFDM signal are orthogonal to each other in both the time and frequency domains, and as such the subcarriers do not interfere with each other. For a given data rate, an OFDM symbol is longer than a symbol from a conventional communications scheme. This helps protect OFDM from multipath effects. In addition to this OFDM employs a cyclic prefix at the beginning of each symbol, which is a copied version of the tail section of the same symbol. This cyclic prefix (also called a guard interval) maintains subcarrier orthogonality and prevents inter-symbol interference.
The 802.11p PHY is similar to the 802.11a PHY with two primary differences, namely, the 802.11p standard uses a 10 MHz bandwidth, where the 802.11a standard uses a 20 MHz bandwidth, and the 802.11p standard uses an operating frequency of 5.9 GHz, where the 802.11a standard uses an operating frequency of 5 GHz. When using a binary phase-shift keying (BPSK) modulation scheme with ½ coding rate, this yields a data rate of 3 Mb/s.
The V2V environment is outdoor and highly dynamic. Thus, the channel characteristics of V2V channels are fundamentally different from those of indoor stationary channels. By directly adopting a standard 802.11p transceiver the communications system may be unreliable. Therefore, understanding the V2V channels to develop wireless transceivers that are particularly suited for V2V channels is a prerequisite to realizing reliable V2V communications that will enable the envisioned applications.
Initial research focused on studying the behavior of the mobile V2V channel. This research measured the statistical characteristics of the V2V channel, and studied the feasibility of using different time scaled OFDM waveforms. The efforts identified that the primary detriment to performance of the 802.11p standard is the channel's short coherence time. Because, the 802.11p standard does not restrict the length of data packets, the short coherence time is a major concern. Short packets will naturally have better performance, whereas longer packets will suffer from the short coherence time of the channel.
Previous research focused on designing receiver technologies within the 802.11p standard. Those efforts produced several equalization schemes for OFDM including Spectral Temporal Averaging (STA) and the Triple Decoding scheme. However, the best achievable packet error rate, PER, was still only about 15%, which fails to provide reliable V2V communications, which comes with a PER of 10% or less.
What is needed is a better transceiver that can achieve a very low PER to provide a reliable vehicular communications network.